400 CLINICAL BACTERIOLOGY. 



C. Frankel and Pief ke. At the bottom of a filtering basin 

 large cobblestones should be placed to a height of 305 mm., 

 upon these a layer of small cobblestones, 102 mm. high, 

 then 76 mm. of coarse gravel, 127 mm. gravel of medium 

 size, upon this 51 mm. of coarse sand, and, finally, 559 mm. 

 of fine sand. Filtering properties reside solely in the layer 

 of sand. Before the process of filtration is begun the 

 reservoir must be permitted to remain filled with water for 

 twenty-four hours. As a result, there will form a coating 

 of sediment, and a slimy covering for the pores of the 

 filter, which constitute the essential factors in the purifica- 

 tion of the water. The rate of filtration should not ex- 

 ceed 100 mm. an hour. The form of filter described does 

 not yield perfectly sterile water, but this will be found to 

 contain between 50 and 200 germs in the cubic centimeter, 

 which it must be noted are derived in largest part from the 

 lower layers of the filter. The mechanism can be disturbed 

 by tears in the cover of the filter, when the filtration- 

 pressure, which will increase with increasing sliminess of 

 the filter, becomes too great. The filter must, therefore, 

 be cleansed from time to time. The working of the filter 

 altogether demands unremitting attention. The water of 

 every filter must be examined bacteriologically every day, 

 and as soon as more than 100 bacteria are contained in the 

 cubic centimeter, use of the water should not be permitted, 

 and the filter should be renewed. 



Of house-filters for the purification of water for domestic 

 use numerous varieties have been recommended. Of all 

 not one properly serves its purpose, as they, are soon sat- 

 urated with bacteria. For domestic purposes suspected 

 water is rendered harmless in the simplest manner by boil- 

 ing for five minutes (after the vapor of steam has arisen). 

 Of wells the best as a source of water-supply are of the 

 tubular variety, in which an iron tube leads to the subsoil- 

 water area, and thus furnishes germ-free water. 



Ice. Natural ice, which is obtained in winter from 

 streams and ponds, contains numerous bacteria on an 

 average 2000 to the cubic centimeter in ice-water, with a 

 minimum of 50 and a maximum of 25,000 germs. Quite 

 a number of bacteria, among them the cholera-vibrios, offer 

 considerable resistance to freezing. Some even are capable 

 of multiplication at this temperature (p. 22). Artificial ice 

 prepared from distilled water contains from none to 10 



